Corona, the outermost layer of the Sun’s atmosphere. They are made up of plasma. Like the chromosphere, the corona can be observed only during total solar eclipse − when the Moon completely covers the solar disc (Fig. below).
Q. why we cannot see corona at normal times?
The fact is that the density of matter in both the chromosphere and corona is very low. They emit very little light and, as a result, they are very faint. In the bright light of the photosphere, they are not visible.
- The spectrum of corona consists of bright lines superimposed on a continuous spectrum.
- When these lines were first discovered, they were thought to be due to a new element, coronium, not found on the earth. Later, it was realized that these lines were due to highly ionised atoms and not due to the so-called ‘new’ element coronium. the temperature and height in the corona at which emission lines of various ionised elements are formed. The observed emission lines of highly ionised atoms of iron, nickel, neon, calcium etc., in the spectrum of corona clearly indicate that the temperature prevailing in corona is very high (more than 1000000 K).
- Due to high temperature, electrons in the corona region have high energies. These electrons interact with ionised atoms and give rise to emission of X-rays. The coronal X-ray emission is much larger than that of the photosphere.
- Remember that the
temperature of the photosphere is only 6000 K. So, it emits very little energy in the
X-ray region. the temperature of the photosphere is lower than that of
the chromosphere and as one goes further up in the corona, temperature rises to more
than a million degree K.
Q Despite being closer to solar interior, why is the photosphere far cooler than the corona?
You know that the second law of thermodynamics precludes such a scenario as heat cannot flow from a cooler region to a hotter region on its own. We also know that the radiation from the photosphere passes through corona almost freely because of its (corona’s) low density. Since hardly any absorption of radiation takes place in the corona, the existence of such high (~ million degree) temperature in the corona presents a paradoxical situation. Several mechanisms have been proposed to resolve the paradox. It is now generally believed that the magnetic field of the Sun might, in some way, be responsible for coronal heating. The observed overlapping of regions of intense X-ray emission and strong magnetic fields lend support to this idea.